1. Field of the Invention
The present invention relates to a spark ignition internal combustion engine, and more particularly to a spark ignition internal combustion engine that is equipped with a port fuel injector and direct fuel injector.
2. Background Art
A conventionally known spark ignition internal combustion engine is disclosed, for instance, by Japanese Patent Laid-Open No. 293304/1995. This conventional spark ignition internal combustion engine comprises a port fuel injector for injecting fuel into an intake port of an internal combustion engine and direct fuel injector for directly injecting fuel into a cylinder of the internal combustion engine. During low-to-medium load operation, this conventional internal combustion engine invokes stratified combustion by supplying fuel according to direct injection technique. During high load operation, on the other hand, the conventional internal combustion engine invokes homogeneous combustion by supplying fuel according to a port injection technique.
When the intra-cylinder injection technique is used, an air-fuel mixture layer of high fuel concentration can be formed near a spark plug. In this instance, proper ignition can be achieved with a small amount of fuel; therefore, a stable operation can be performed at a lean air-fuel ratio. When the port injection technique is used, on the other hand, an air-fuel mixture of uniform concentration can be introduced into a cylinder. Consequently, a high output can be generated while stable combustion is maintained. As a result, the above-mentioned conventional internal combustion engine provides enhanced fuel efficiency and excellent operation characteristic.
However, when injected by the port injection technique, the fuel does not completely vaporize within the intake port and partly adheres to the wall surfaces of an intake valve and intake port. It is likely that various factors, including the fuel adhesion distribution and the air flowing upon valve opening, determine how the fuel adhering to the wall surface flows into the cylinder. Therefore, if the port injection technique is solely used, the air-fuel mixture within the cylinder cannot always be rendered homogeneous.